The increasing needs of understanding the processes of groundwater contamination have necessitated the development of new, sampling techniques. In hydrochemical studies it is important to obtain small-interval-profiles of the examined water layer. Generally water samples are collected from pumping wells or by samplers lowered to different depths in research wells. These procedures disturb chemical gradients and yield only mixed water samples from different levels of the aquifer. In literature there have been described samples which have been used primarily for shallow aquifers. In most of these devices the water sample is pumped from the surface and the sampling intervals are of the order of 0.5 m.
Samplers developed for water studies in lakes employ the dialysis membrane technique which consists of placing a sampler having dialysis cells filled with distilled water at the sampling site and leaving it there for equilibration between the ambient water and the cell water. Sampling intervals of 1 cm have been achieved using this method.
The equilibration time of a dialysis cell system can be calculated using Fick's second law of diffusion.